Run `make obj` before running `make test-includes`

[ngie]

Summary

Before this change, make test-includes (run as part of buildworld)
would place test files in the current directory, which would clutter up
git clones. Run make obj beforehand to ensure that the files are put
in ${.OBJDIR} instead of ${.CURDIR}. This helps cut down on the
noise significantly when running commands like git status.

MFC after: 1 week

Test Plan

Running git status after running make test-includes from the top-level now shows something like the following:

% git status tools/build/test-includes 
On branch release-sh-changes
Your branch is ahead of 'freebsd/main' by 3 commits.
  (use "git push" to publish your local commits)

nothing to commit, working tree clean
%